1. Field of the Invention
This invention relates to hydraulic fracturing of earth formations, and more particularly to the hydraulic fracturing of hydrocarbon bearing formations, i.e., oil and gas sands, for the purpose of increasing the producing rate and total amount of recovery of the hydrocarbons from a well completed in such a formation.
2. Brief Description of the Prior Art
Hydraulic fracturing techniques for hydrocarbon formations are well known and have been extensively used for increasing the recovery of oil and gas from hydrocarbon bearing formations. These techniques involve injecting a fracing fluid down the well bore and into contact with the formation to be fractured. Sufficiently high pressure is applied to the fracing fluid to initiate and propagate a fracture into the formation. Propping materials are generally entrained in the fracing fluid and deposited in the fracture to maintain the fracture open during production.
The function of fracturing is to overcome the deficiency in permeability of the formation adjacent the well bore by creating a highly conductive path reaching out into the producing formation sand and/or rock surrounding the well bore. According to the usual practice, a fluid, such as water, oil, oil/water emulsion, gelled water or gelled oil is pumped down a well bore with sufficient pressure to open a fracture in the formation. The fracing fluid may carry a suitable propping agent, such as sand, glass beads, etc., for the purpose of holding the fracture open after the fracturing fluid has been recovered, e.g., allowing the well to flow. In the case of tight or low permeability wells, that is, wells below one millidarcy permeability, prior art methods of fracturing have produced results that are of but a temporary nature as far as increasing the rate of flow is concerned. After perhaps a short period of accelerated flow, rates of production may drop off to near previous levels. Repeated stimulation with the same or similar procedure may again produce but a temporary gain.
One of the reasons for such a lack of results in a low permeability formation is that at the depths encountered most formations have a preferred vertical fracture orientation which exists because of naturally occurring planes of weakness in the formation as the fracture is formed and are propagated along these planes of weakness. It has been found that these vertical fractures are most advantageous in formations having a relatively wide pay zone and a permeability on the order of 10 to 20 millidarcys.
Unfortunately, many geological oil and gas bearing formations, including some West Texas formations, which are primarily gas formations, comprise multiple, vertically-spaced narrow pay zones, that is 10-to-30 foot pay zone formations, each separated generally by a shale layer. Further, the pay zones are formed in sandstone and have a very low permeability, on the order of 10 to 0.1 millidarcy or less. To further complicate recovery, the pay zones contain contaminants, such as water sensitive clays and iron, which react unfavorably with acids often used in treating the formation.
Using a conventional fracing process, vertical fracturing occurs as above-described. In a gas well of the above-described multiple pay zone type, this results in radial vertical fracturing that extends between the pay zones and through the intervening shale zones. As a result, fracing fluid is lost into the shale zones with no resulting benefit in fracturing the hydrocarbon pay zones.
Additionally, only small vertically oriented radial fractures are created in the pay zones and because of the deep vertical orientation do not permit much radial penetration horizontally into the pay zone itself. A temporary increase in production produced by the vertical fractures is believed to be the result of the fracture permitting communication between the well bore in a small portion of a joint system between the matrix elements of the formation and with a small portion of the reservoir matrix. However, as soon as this low volume space has been drained, productivity drops off to that controlled by the low permeability reservoir matrix, and since the formation area exposed to such matrix by the short radial vertical fractures is small, productivity is low.
The present invention overcomes the disadvantages of the prior art by providing a method for fracturing a producing formation to produce long vertical linear fractures extending outward from a borehole within the zone of interest, with a minimization of radial vertically oriented fractures occurring above and below the producing zone of interest.